This invention relates to a method of cutting webs into a specified length and a device for practicing the method, in which a web is run continuously and cut into a specified length at high speed and with high accuracy.
The term "web" as used herein is intended to mean relatively long, flexible, belt-shaped articles such as plastic films made of polyvinyl chloride, polycarbonate, acrylonitrile styrene copolymer, ABS resin, polyester, polyester resin containing glass fibers, cellulose derivative or the like, or sheets such as paper and synthetic paper, or metal foils of aluminum, copper or the like.
Such a web is generally wound on a roll during its manufacturing process. The web thus wound is subsequently unwound to be cut into a desired length depending on its object of use in the following processing. In this case the method and apparatus for cutting the web into a desired length is especially important. Accordingly, the following two techniques have been extensively employed: In the first method, the web feeding roll and the cutting edges are driven by a single electric motor, and the cutting dimension is set by changing the speed change gear ratio of the reduction gear, for cutting the web into a desired length. In the second method, a clutch is employed instead of the reduction gear, and the cutting edges are driven through the clutch operated by a signal representative of a web running length, to thereby cut the web.
In these mechanical control type web cutting methods in which the cutting timing of the cutting edges is set up by a power transmission unit such as a reduction gear or a clutch, fluctuations or variations between the web running speed and the cutting speed of the cutting edges may be repeatedly induced by mechanical errors or inherent properties involved in the driving system. These include clutch slipping and gear backlash. Accordingly, it is impossible to cut a web into a desired length with repeated high accuracy.
In order to overcome these difficulties, a so-called motor control type web cutting method has been provided in which, as disclosed by Japanese Patent Application Laid-Open No's 890/1974 and 142785/1976, an electric motor for driving the cutting edges is provided in addition to an electric motor for the web feeding roll. The first motor is driven by a web running length signal so that the cutting speed and cutting position of the cutting edges are controlled by the operation of that motor. In this method, the above-described mechanical errors can be generally eliminated which therefore contributes to improvement of the cutting accuracy. Also the cutting length can be set electrically. Thus, this method is one which offers improved performance when compared with the conventional prior art methods described before. However, this method is still disadvantageous in that it is difficult to completely synchronize the cutting speed with the web feeding speed. Therefore, the method is not applicable to this case where it is required to cut a web more precisely with an allowable tolerance.+-.500.mu. or less.
The reason for this is as follows: The first Japanese Patent Application discloses a cutting edge rotation type web cutting method in which a pair of rotary cutting edges confront each other through a web, while the second Japanese Patent Application discloses a cutting edge swinging type web cutting method in which two cutting edges confronting through a web are moved vertically and are moved in the web running direction. Hence on this point, the two Japanese Patent Applications are different from each other. However, in each of them, the speed of the cutting edges in the web running direction is given by circular motion, and therefore, as will become clear from an explanation described later with reference to FIG. 2, the speed of the cutting edges in that direction is greatly varied. Accordingly it is difficult to cut the web with a high degree of accuracy. If this variation of the speed is decreased, the vertical movement of the cutting edges is accordingly also decreased. With this technique, accordingly, it is impossible to provide sufficient shearing angle and engagement depth for the cutting edge, and it is therefore difficult to cut the web sharply. This leads to an irregular configuration in the web cut surface. This drawback may be eliminated by increasing the rotation radius of the cutting edges carrying out the circular motion. With this modification, the web may be cut into longer pieces; however, it is impossible to cut the web into shorter pieces shorter than 500 mm.
Japanese Patent Application Laid-Open No. 160892/1975 describes a web cutting method in which the speed of cutting edges coincides with a web feeding speed. More specifically, in this method, the rotational speed of a pair of spiral rotary cutting edges confronting each other through a web is allowed to synchronize with the web feeding speed. Web cutting is carried out with a constant maintained speed of the cutting edges in the web feeding direction.
It is, however, difficult to manufacture such spiral cutting edges. Also, it is hard to adjust the edges of the cutting edges, and therefore difficult to cut the web sharply. Furthermore, even if the web can be sharply cut, cutting the web into shorter pieces cannot be easily accomplished. Hence, this technique also has serious disadvantages.